1. Field of the Invention
The invention relates generally to a phase-locked loop (PLL) and, more particularly, to an improved charge pump with transient current correction.
2. Description of the Related Art
High-performance, low-jitter phase-locked loops (PLLs) require accurate sensing and correction of the phase and frequency error between a reference clock signal and a feedback clock signal. Typically, a PLL includes a phase-frequency detector (PFD), a charge pump, a loop filter, a voltage-controlled oscillator (VCO), and optionally a frequency divider. The PFD senses the aforementioned phase and frequency error and generates timing signals, which are used to generate currents in the charge pump. These currents are then integrated by a loop filter to create a control voltage, which is input to the VCO. This voltage controls the frequency of the VCO. Ideally, a plot of this control voltage as a function of a phase error should produce a linear response over the cycle and should pass through the origin of the plot. For conventional charge pumps currently used in PLLS, however, this is not the case.
Conventional charge pumps have current source and sink along with switching devices used to control current flows through the current source and sink. The output signals of a PFD switch these switching devices. Typically, when the feedback clock signal leads the reference clock signal, the current sink is coupled to the loop filter so that the control voltage is decreased. When the reference clock signal leads the feedback clock signal, the current source is coupled to the loop filter so that the control voltage is increased. When the PLL is locked, nether the current source nor the current sink is coupled to the loop filter so that the control voltage does not change.
The switching devices are coupled to the current source and sink in series. The devices may be positioned either at the top and bottom of the current source and sink or between the stack of the current source and sink. In such structures, the switching devices force the current in the current sources or sinks to be shut off, resulting in large biasing differences between conducting and non-conducting states. The current source and sink are typically implemented with current mirrors. The current mirrors generally comprise a plurality of transistors such as metal-oxide-silicon field effect transistors (MOSFETs). These three-terminal transistors have parasitic capacitances between a gate and the other two terminals (drain and source terminals in case of MOSFETs). These parasitic capacitances contribute additional transient currents, which distort the linearity of the control voltage as a function of a phase error when the phase error is small. For small phase errors, the initial transient current dominates the charge pump's response. In the aforementioned plot of the control voltage as a function of a phase error, therefore, a conventional charge pump would generate a higher slope region at or near the origin where the transient currents flow and a discontinuity in the response after the transient currents have died out. There is also another discontinuity at the origin of the transfer function because the transient currents are different for charge and discharge operations. These transient currents are not well controlled since they are due to the design and process of the switching devices.
Therefore, there is a need for an improved charge pump that eliminates these transient currents associated with the switching devices used in the charge pump.